[alsa-devel] [PATCH alsa-lib v2 5/6] pcm: hw: add a helper function to issue avail_min without side-effects

[Takashi Sakamoto]

At present, applications can change avail_min parameter of PCM substream
by two ways; via mapped control data, and by executing ioctl(2) with
SNDRV_PCM_IOCTL_SYNC_PTR. The former is available in a case that the
applications map the data successfully.

When utilizing alsa-lib API, the above is done by a call of
'snd_pcm_sw_params()' to hw PCM plugin. In current implementation, this
call has an side-effect to issue appl_ptr unexpectedly.

This commit adds a helper function to issue avail_min without the
side-effect.

Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
 src/pcm/pcm_hw.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/pcm/pcm_hw.c b/src/pcm/pcm_hw.c
index 51512306..9c8fc3f6 100644
--- a/src/pcm/pcm_hw.c
+++ b/src/pcm/pcm_hw.c
@@ -151,6 +151,15 @@ static int sync_ptr(snd_pcm_hw_t *hw, unsigned int flags)
 	return 0;
 }
 
+static int issue_avail_min(snd_pcm_hw_t *hw)
+{
+	if (!hw->mmap_control_fallbacked)
+		return 0;
+
+	/* Avoid unexpected change of applptr in kernel space. */
+	return sync_ptr1(hw, SNDRV_PCM_SYNC_PTR_APPL);
+}
+
 static int issue_applptr(snd_pcm_hw_t *hw)
 {
 	if (!hw->mmap_control_fallbacked)
@@ -506,7 +515,7 @@ static int snd_pcm_hw_sw_params(snd_pcm_t *pcm, snd_pcm_sw_params_t * params)
 	    params->silence_size == pcm->silence_size &&
 	    old_period_event == hw->period_event) {
 		hw->mmap_control->avail_min = params->avail_min;
-		return sync_ptr(hw, 0);
+		return issue_avail_min(hw);
 	}
 	if (params->tstamp_type == SND_PCM_TSTAMP_TYPE_MONOTONIC_RAW &&
 	    hw->version < SNDRV_PROTOCOL_VERSION(2, 0, 12)) {
-- 
2.11.0